The invention relates generally to a thermal print head and more particularly to a thermal print head in which the heat generating element is disposed close the edge of a heat resistant substrate.
A conventional thermal print head shown generally in FIGS. 5, 6 and 7 typically has the structure of a thermal print head 50, 60 or 70, respectively with similar structures assigned the same reference numerals. Print heads 50, 60 and 70 can be used for serial type or line type printing. Print head 50 includes a heat resistant substrate 11 having a glass glaze layer 12 disposed thereon and a heat generating element 13 disposed on glass layer 12 and substrate 11. A common electrode 14 is disposed on heat generating element 13 in a region E of FIG. 5 along both the lower and upper side of glass layer 12. An independent electrode 15 is disposed on another portion of heat generating element 13. A passivation layer 16 is disposed over all of these elements, on common electrode 14, independent electrode 15 and on an exposed portion of heat generating element 13.
A second conventional thermal print head 60 is shown in cross-sectional view in FIG. 6. Common electrode 14 of print head 60 is formed down the side edge of substrate 11 and around to the underside of substrate 11. This construction permits common electrode 14 to be provided of larger size which reduces the electrical resistance of electrode 14.
A third conventional structure for a thermal print head is shown generally as print head 70 and FIG. 7. Common electrode 14 of print head 70 is disposed between partial glass glaze layer 12 and heat resistant substrate 11 and continues over a portion of heat generating element 13. Providing common electrode 14 underneath glass glaze layer 12 permits electrode 14 to be larger which increases the current capacity of common electrode 14.
As illustrated in FIG. 8, when partial glass glaze layer 12 is provided on a bottom surface of a print head substrate 21, a print head 80 will typically form an angle .alpha..sub.1 relative to the surface of a recording medium 23 and a printing ribbon 22 will typically form an angle .alpha..sub.2 with recording medium 23. By providing partial glass glaze layer 12 on a bottom surface near an edge of substrate 21, it is possible to obtain a large angle .alpha..sub.1 and .alpha..sub.2. Large angles .alpha..sub.1 and .alpha..sub.2 permit the force from print head 80 pressing into ribbon 22 and recording medium 23 to be concentrated at a small point to improve print quality for both serial type and line type printing.
Although it is desirable to position the heat generating element on a glass layer close to the edge of the print head, such a configuration leads to certain disadvantages.
1. The region for securing the common electrode is narrow and the common electrode is thereby small;
2. The current capacity of a small sized common electrode is low and when many dots are energized, the voltage drop from the small common electrode deteriorates print density and quality;
3. If a driving method employing an o'clock/minutes driving method is employed to compensate for the deterioration and print density, print speed is decreased and the necessary control mechanisms become more complicated which increases costs;
4. If the common electrode is provided along a side surface of the print head substrate, as shown in FIG. 6, costs for manufacturing the print head increase significantly;
5. To form print heads having the configurations or print heads 50 and 60, there should be about 200 to 300 .mu.m between the edge of heat resistant substrate 11 and the edge of glass layer 12. When the head is formed with a large number of dots, it is difficult to position glass glaze layer 12 as close as is required to the edge of the print head.
A method for overcoming these disadvantages was proposed in Japanese laid open patent application No. 132580/86 the contents of which are incorporated herein by references which describes a print head configured as shown in print head 70 of FIG. 7. Although this configuration compensates for many of the disadvantages of prior art print head 50, it does not provide sufficient print speed and the common electrode lacks sufficient current capacity. U.S. Pat. No. 4,768,038 to Shibata, the contents of which are incorporated herein by reference, also proposes an improved thermal print head, but the print head described therein is also not fully acceptable.
Accordingly, it is desirable to provide an improved thermal print head that does not have the shortcomings of the prior art.